$-\dfrac{3}{10} + \dfrac{3}{4} = {?}$
$ = - {\dfrac{3 \times 2}{10 \times 2}} + {\dfrac{3 \times 5}{4 \times 5}} $ $ = - {\dfrac{6}{20}} + {\dfrac{15}{20}} $ $ = - \dfrac{{6} + {15}}{20} $ $ = \dfrac{9}{20}$